Human Physiology, 14th edition (2016)

Sensory Physiology 293

CLINICAL APPLICATIONS

Glaucoma is damage to the optic nerve, involving the loss

of retinal ganglion cell axons (see fig. 10.36 ) that results in

a loss of vision that cannot be regained. Most common is

open-angle glaucoma, where the aqueous humor of the

anterior chamber can exit the eye through the “angle,” or

opening, located at the meeting of the iris and cornea. How-

ever, from here the fluid does not drain out properly through

the spongy canal of Schlemm, and so intraocular pressure

can rise above the normal 10 to 21 mmHg. Optic nerve

damage does not correlate perfectly with the intraocular

pressure—people with normal pressure may get glaucoma,

and people with somewhat elevated pressure may not get it.

Open-angle glaucoma is painless, and may lack symptoms

other than the gradual loss of peripheral vision. Its progres-

sion can be treated with medicines and some surgical pro-

cedures. Angle-closure glaucoma, by contrast, which is

caused by the sudden rise in intraocular pressure due to

blockage of the angle by part of the iris, must be treated

immediately with laser surgery to prevent blindness.

The lens is suspended from a muscular process called the

ciliary body, which connects to the sclera and encircles

the lens. Zonular fibers ( zon 5  girdle) suspend the lens from the

ciliary body, forming a suspensory ligament that supports

the lens. The space between the cornea and iris is the anterior

chamber, and the space between the iris and the ciliary body

and lens is the posterior chamber ( fig. 10.29 ).

The anterior and posterior chambers are filled with a fluid

called aqueous humor. This fluid is secreted by the ciliary

body into the posterior chamber and passes through the pupil

into the anterior chamber, where it provides nourishment to the

avascular lens and cornea. The aqueous humor is drained from

the anterior chamber into the scleral venous sinus (canal of

Schlemm), which returns it to the venous blood ( fig. 10.29 ).

The portion of the eye located behind the lens is filled

with a thick, viscous substance known as the vitreous body,

or vitreous humor. Light from the lens that passes through

the vitreous body enters the neural layer, which contains pho-

toreceptors, at the back of the eye. This neural layer is called

the retina. Light that passes through the retina is absorbed by

a darkly pigmented choroid layer underneath. While passing

Figure 10.29 The production and drainage of aqueous humor. Aqueous humor maintains the intraocular pressure within
the anterior and posterior chambers. It is secreted into the posterior chamber, flows through the pupil into the anterior chamber, and
drains from the eyeball through the canal of Schlemm.

Aqueous

humor

Cornea

Lens

Anterior

chamber

Posterior

chamber

Vitreous

humor

Iris

Canal of Schlemm

Ciliary body